The present invention relates to a blanket for an offset printing and a method of manufacturing the same, particularly, to a blanket for an offset printing used in a gapless offset rotary press and a method of manufacturing the same.
FIG. 1 exemplifies a conventional offset rotary press. As shown in the drawing, the press comprises a pair of blanket cylinders 2, 2 having a roll paper (web paper, printing paper sheet) 1 held therebetween, plate cylinders 3, 3 arranged in contact with the blanket cylinders 2, 2, respectively, inking arrangements 4, 4 for supplying an ink to these plate cylinders 3, 3, respectively, and dampening water units 5, 5. The ink is transferred onto the printing paper sheet held between the blanket cylinders 2, 2.
In the offset press of the construction described above, a sheet-type blanket 6 is wound about the blanket cylinder 2, and the both end portions of the blanket 6 are inserted into a gap 7 and fixed by fastening with a fastening tool 8, as shown in FIG. 2. The plate cylinder 3 arranged in contact with the blanket cylinder 2 is also provided with the gap 7.
It should be noted that, in the offset rotary press of the particular construction, printing is not performed in the gap portion of the printing paper sheet. After the printing operation, the gap portion is cut away and discarded. Also, if these blanket cylinder 2 and the plate cylinder 3 are rotated at a high speed, an impact or vibration is generated in the gap portion so as to give rise to dotted slurs (deformation of dotted points in the rotating direction) in the print, thereby making the print defective.
To overcome the above-noted defects, a gapless offset rotary press using a cylindrical blanket has been proposed in, for example, Japanese Patent No. 2569213 and Japanese Patent No. 2519225 (U.S. Pat. No. 4,913,048). Several proposals have also been made in respect of the cylindrical blanket (tubular blanket) used in these gapless offset presses in, for example, Japanese Patent Disclosure (Kokai) No. 5-301483, Japanese Patent Disclosure No. 8-216548, Japanese Patent Disclosure No. 7-1857, Japanese Patent Disclosure No. 7-1858 and Japanese Patent Disclosure No. 6-340187.
These cylindrical blankets have been proposed on the basis of the conventional technique of the sheet-type blanket, and the problems inherent in the conventional sheet-type blanket remain unsolved in the cylindrical blanket. The most serious problem to be noted is that the printed surface is rendered defective by the reduction in the thickness of the cylindrical blanket (sinking), which takes place during use of the cylindrical blanket. In other words, the thickness reduction (sinking) causes the printing pressure to be lowered so as to render the printed surface defective.
An additional problem to be noted is that fine lines arranged immediately below a printing surface rubber are reproduced on the printed surface in the form of a pattern consisting of differences in thickness of the ink. In general, the cylindrical blanket consists of a laminate structure including a nickel sleeve having a predetermined diameter and a thickness of about 0.15 mm, a solid rubber base layer bonded directly to the surface of the nickel sleeve with an adhesive, a compressible porous rubber layer formed on the base layer, an inextensible layer prepared by winding threads or filaments about the compressible porous rubber layer, and a surface rubber layer formed on the inextensible layer.
The gapless offset rotary press is mainly intended to reduce the printing cost by markedly increasing the printing rate by a high speed rotation and, thus, is required to achieve a high working rate. Naturally, the cylindrical blanket is required to exhibit a stability higher than that in the conventional sheet-type blanket. It should be noted in this connection that, where thickness reduction takes place in the conventional sheet-type blanket, it is impossible to re-use the blanket unless the underlying plastic film is replaced by a thicker one. In the cylindrical blanket, however, the blanket is formed integral with the support body of the sleeve, making it impossible to replace the rubber layer alone. It follows that the thickness reduction in question makes it impossible to repair the cylindrical blanket so as to be used again.
In the sheet-type blanket, plain weave fabric is used as an inextensible layer. In the cylindrical blanket, however, an inextensible layer prepared by continuously winding a thread or a filament is arranged between the compressible layer and the surface rubber layer. Also, the thread used for forming the inextensible layer consists of a cotton thread or a synthetic thread having a diameter of 0.1 to 0.5 mm. The particular thread is helically wound from one end of the sleeve to form the inextensible layer.
A serious problem inherent in the cylindrical blanket thus prepared is that, since the printing speed of the gapless offset press using the cylindrical blanket is markedly higher than that for the conven- tional offset web press so as to increase the printing productivity, the thickness reduction is brought about promptly in the cylindrical blanket to make the cylindrical blanket unusable in a shorter time, compared with the conventional sheet-type blanket. Also, a striped irregularity is formed on the surface rubber layer, and a striped pattern consisting of differences in thickness of the ink and conforming with the striped irregularity on the surface rubber layer is formed on the printed paper sheet to make the printing unsatisfactory.